Method of manufacturing an oxidation-resistant component of a molybdenum base alloy

ABSTRACT

The present invention relates to a method of producing a component of an Mo base alloy which is protected against high-temperature oxidation, and a correspondingly produced component. 
     The method comprises:
         provision of a semifinished part composed of a Mo base alloy,   provision of an Si-containing slip or of a Si-containing powder,   application of the slip to the semifinished part and diffusion annealing of the semifinished part together with the applied slip to form a Si-containing outer layer or   transfer of at least part of the silicon present in the powder via the gas phase to the semifinished part by means of a diffusion heat treatment of the semifinished part together with the Si-containing powder which is arranged at a distance from the semifinished part.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 of GermanPatent Application No. 102018215313.2, filed Sep. 10, 2018, the entiredisclosure of which is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method of improving thehigh-temperature oxidation resistance of molybdenum-based alloys ormolybdenum base alloys. In addition, the present invention relates to acorrespondingly produced component composed of a molybdenum base alloywhich can, in particular, be a component of a turbomachine and inparticular an aircraft engine.

2. Discussion of Background Information

Molybdenum and its alloys are interesting materials for high-temperatureapplications because of the high melting point and the good corrosionresistance. However, in order to avoid high-temperature oxidation it isnecessary to enrich at least the surface regions of a correspondingcomponent composed of molybdenum or a molybdenum-based alloy withsilicon to such an extent that the silicon together with oxygen forms aslowly growing silicon oxide layer which protects the component againstfurther oxidation.

For this purpose, subjecting components composed of molybdenum-basedalloys to a diffusion heat treatment in a powder packing composed ofsilicon-containing powder, so that the regions of the molybdenum-basedcomponent which are close to the surface can be enriched in silicon bydiffusion processes, is already known. However, the known powder packmethod has the disadvantage that it requires very long processing times.Furthermore, caking of the powder particles which surround the componentcan occur in the diffusion heat treatment, with the result that thecomponent has to be released from a solid shell after the correspondingdiffusion heat treatment, which can lead to damage to the component. Inaddition, the removal of the caked silicon-containing powder istime-consuming.

It would therefore be advantageous to have available a method by meansof which a molybdenum-based component resistant to high-temperatureoxidation can be produced in a simpler manner compared to the prior art,with, in particular, improved process efficiency being ensured. However,the component should also be reliably protected against high-temperatureoxidation.

SUMMARY OF THE INVENTION

The present invention provides a method having the features of theindependent method claim. A component having the features of theindependent product claim is likewise provided by the present invention.Advantageous embodiments are subject matter of the dependent claims.

In order to solve the problems arising in the prior art, the inventionproposes applying the silicon via either a liquid or a gaseous phase toan appropriate region to be protected against high-temperature oxidationon a molybdenum-based semifinished part in order to produce acorrespondingly protected component. Accordingly, the method of theinvention encompasses not only the provision of a molybdenum-basedsemifinished part but also the provision of either a silicon-containingslip or a silicon-containing powder.

The silicon-containing slip is applied to the regions of thesemifinished part which are to be protected against high-temperatureoxidation, with diffusion annealing subsequently being carried out sothat silicon can diffuse from the slip into at least the regions closeto the surface of the molybdenum-based semifinished part in order toprovide silicon there for formation of a silicon oxide layer.

The silicon-containing powder is provided in order to introduce siliconinto the region close to the surface of the semifinished part via thegas phase. However, instead of arranging the semifinished part in apowder packing, the silicon-containing powder is arranged at a distancefrom the semifinished part to be protected, so that the silicon candiffuse into the surface of the semifinished part in a diffusion heattreatment on the semifinished part with the silicon-containing powder.

After the regions close to the surface of the semifinished part havebeen enriched in silicon, the semifinished part prepared in this way canadditionally be conditioned by means of an oxidation treatment, so thata thin, slowly growing silicon oxide layer which protects the componentagainst further oxidation is formed on the component. The conditioningcan be carried out by means of an oxidation treatment at a temperatureof more than about 900° C., in particular in the temperature range fromabout 1000° C. to about 1400° C., in particular at about 1380° C., for atime of up to about 2 to 100 hours, preferably from about 10 to 15hours, in particular about 12 hours.

Heating to the treatment temperature and cooling from the treatmenttemperature can be carried out slowly, in particular at a heating and/orcooling rate of less than or equal to about 10 K/min.

Conditioning can be carried out in ambient air or under specificallyprepared oxygen-containing gases.

As semifinished part, it is possible to use, for example, a semifinishedpart composed of Mo alloys with Si and/or titanium and/or boron and/orFe. The proportion of Si can here be in the range from about 5 to about25 at. %, while Ti can be present in the alloy in an amount in the rangefrom 0 to about 30 at. % and B can be present in an amount in the rangefrom about 5 to about 15 at. %. In addition, such an Mo base alloy cancomprise up to about 5 at. % of iron, with any combinations of thealloying elements being possible while the respective balance is formedby Mo.

The enrichment of at least subregions of the surfaces of thesemifinished part with silicon via application of a slip or via the gasphase is, in the present invention, carried out in particular directlyon the above-described materials of the semifinished parts, so that noadditional intermediate layers on the surface of the semifinished partare necessary.

In the diffusion heat treatment for transferring the silicon via the gasphase from the silicon-containing powder into the molybdenum-basedsemifinished part provided, halogens can be present in thesilicon-containing powder in order to improve the diffusion of thesilicon into the semifinished part. As halogen-containing compounds, itis possible to use NH₄F, NH₄Cl or NaF.

Furthermore, the silicon-containing powder for transferring the siliconvia the gas phase into the semifinished part can contain additionalconstituents in addition to the silicon powder. In particular, it ispossible to use a mixture of silicon powder and aluminum oxide powder.The additional constituents of the powder can serve as filler materialand prevent caking of the powder. Furthermore, additional constituentscan influence the total amount of silicon and control the gas-phaseactivity of the silicon. The silicon-containing powder for transferringthe silicon via the gas phase into the semifinished part can preferablybe arranged in a ceramic vessel underneath the semifinished part duringthe diffusion heat treatment.

The diffusion heat treatment of the semifinished part with thesilicon-containing powder arranged at a distance can be carried out at atemperature of more than about 900° C., in particular in the temperaturerange from about 1100° C. to about 1300° C. The hold time at theappropriate heat treatment temperature can be in the range from about0.5 to 5 hours and preferably in the range from about 1 to 2 hours. Forthe diffusion heat treatment, the semifinished part can be arrangedtogether with the powder in a protective gas atmosphere, for example anargon atmosphere or a hydrogen atmosphere.

The slip can comprise silicon-containing powder or silicon powder and asolvent and a binder.

Possible solvents are, for example, water, alcohols or alcoholicsolvents or liquid-organic solvents. As binders, it is possible to use,for example, polyvinyl alcohols or resins. The slip can contain furtherconstituents such as Mo, W, B, Ta, Cr, Fe, Ti and alloys thereof, withthese components either being able to be present as alloyingconstituents in the silicon-containing powder or being able to be addedas separate powder particles. For example, constituents for controllingthe silicon activity or matching the coefficients of thermal expansionof the layer produced and of the substrate or semifinished part can beadded.

Furthermore, the slip can contain further constituents in the form ofoxide, carbide or nitride particles which can be incorporated into asilicon oxide layer on the component in order to reduce the viscosity ofthe oxide layer and prevent running-off of the oxide layer at high usetemperatures of the component. Accordingly, the slip can containaluminum oxide, zirconium oxide, yttrium oxide, hafnium oxide, neodymiumoxide, silicon carbide and/or silicon nitride.

The powder particles can be present with an average particle size or amaximum particle size of from about 0.5 μm to about 100 μm and inparticular about 1-60 μm in the slip.

The slip can be applied to the semifinished part by dipping, spraying,printing and in particular by screenprinting or template printing.

The diffusion annealing of the semifinished part with the applied slipcan be carried out at temperatures above about 900° C. and in particularat temperatures of from about 1000° C. to about 1400° C. The hold timeat the annealing temperature can be in the range from about 1 to 3 hoursand in particular up to about 2 hours.

Definitions

When indications of position are given in the present description, forexample bottom, top or the like, these indications are based on theusual position during use in the gravitational system of the Earth, sothat top indicates the position remote from the Earth's surface, whilebottom is localized in the direction toward the Earth's surface.

For the purposes of the present invention, a molybdenum-based alloy ormolybdenum base alloy is an alloy whose largest constituent ismolybdenum. Alloys in which one alloying component is present in anequal or similar proportion to that of molybdenum in the alloy are alsointended to be included under the term molybdenum-based alloy ormolybdenum base alloy. Accordingly, the term molybdenum-based alloys ormolybdenum base alloys refers to alloys which comprise more than 50percent by weight or atom percent of molybdenum.

For the purposes of the present invention, high-temperature oxidation isan oxidation at temperatures which are higher than normal ambienttemperatures and in particular higher than about 500° C., preferablyhigher than about 1000° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings schematically show in

FIG. 1 a depiction of an arrangement for producing an Si-containingouter layer via the gas phase,

FIG. 2 in subfigures a) and b), a depiction of the application of a slipto a semifinished part,

FIG. 3 a depiction of a cross section through the outer region of asemifinished part which has been treated as shown in FIG. 1 or FIG. 2,and in

FIG. 4 a depiction of a cross section through the outer region of thesemifinished part of FIG. 2 after conditioning.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show details of the present invention in more detail than isnecessary for the fundamental understanding of the present invention,the description in combination with the drawings making apparent tothose of skill in the art how the several forms of the present inventionmay be embodied in practice.

In various working examples, technical-grade molybdenum, a molybdenumalloy comprising 9 at. % of silicon and 8 at. % of boron with molybdenumas balance and also a molybdenum alloy comprising 27 at. % of titanium,13.5 at. % of silicon, 5.5 at. % of boron and 1 at. % of iron withmolybdenum as balance have been used as materials of the treatedsemifinished parts. In the case of Mo base alloys, oxides such as La₂O₃can additionally be added to the material. An overview of alloys fromwhich the semifinished part can be made is given in the following table:

Proportion in at. % alloy No. Mo Si Ti B Fe Nb Hf Al Cr W V 1 Balance 98 2 Balance 13.5 27 5.5 1 3 Balance 9 8 2.7

FIG. 1 shows an example of an arrangement for producing an Si-containingouter layer in a semifinished part 4 via the gas phase. For thispurpose, a vessel 1, for example made of aluminum oxide, which has a lid2 and in which a silicon powder containing NH₄F is present is provided.The semifinished part 4 to be treated is located above the siliconpowder so that when the vessel 1 is heated to a temperature of about1190° C. in an argon atmosphere for a time of 2 hours, silicon candiffuse into the outer layer of the semifinished part 4.

The diffusion heat treatment results in formation of an outer region ofthe semifinished part 4 as is shown in cross section in FIG. 3. Asilicide layer 6 has been formed on top of the base material 5 by inwarddiffusion of silicon.

After the formation of the silicide layer 6 by means of the heattreatment under an argon atmosphere, as has been shown schematically inFIG. 1, the semifinished part 4 is aged in air at a temperature of about1400° C. for 8 hours so that a silicate layer 8 is formed on top of thesilicide layer 6 by oxidation of the silicon, while an interdiffusionlayer 7 which serves as silicon reservoir is formed between the silicidelayer 6 and the base material. This structure of the outer region isshown in FIG. 4.

FIG. 2 shows, in the subfigures a) and b), an alternative workingexample in which the silicide layer 6 is formed by a slip 12, which ispresent in a vessel 10, being applied by means of a brush 11 to thesemifinished part 4 and drying at about 50° C. subsequently beingcarried out. A heat treatment under reduced pressure at 1400° C. for 1hour is subsequently carried out so that silicon can once againpenetrate into the base material 5 of the semifinished part 4 and asilicide layer 6, as depicted in FIG. 3, is formed. The semifinishedpart 4 which has been enriched with silicon is appropriately conditionedso that formation of the outer region as depicted in FIG. 4 and as hasbeen described above occurs.

The enrichment with silicon by means of the slip process can of coursebe applied to all molybdenum-containing materials in the same way as thesilicon gas-phase coating procedure.

In one working example, the slip is formed by a silicon powder having aparticle size of 45 μm in an aqueous solution, with additionalcomponents, for example boron powder having a particle size of 35 μm orthe like, being able to be added to the slip solution.

Although the present invention has been described in detail with the aidof the working examples, it will be self-evident to a person skilled inthe art that the invention is not restricted to these working examplesbut instead that modifications made by omitting individual features ordifferent combinations of features may be made without going outside thescope of protection of the accompanying claims. In particular, thepresent disclosure includes all combinations of the individual featuresindicated in the various working examples, so that individual featureswhich have been described only in connection with one working examplecan also be used in other working examples or combinations of individualfeatures which are not explicitly presented.

1.-13. (canceled)
 14. A method of producing a component of a Mo basealloy which is protected against high-temperature oxidation, wherein themethod comprises: provision of a semifinished part composed of a Mo basealloy, provision of a Si-containing slip which comprises powder of atleast one of Mo, W, B, Ta, Cr, Fe, Ti and alloys thereof or of aSi-containing powder which comprises a mixture of Si and Al₂O₃ powders,(a) application of the slip to the semifinished part and diffusionannealing of the semifinished part together with the applied slip toform a Si-containing outer layer or (b) transfer of at least part of thesilicon present in the powder via a gas phase to the semifinished partby a diffusion heat treatment of the semifinished part together with theSi-containing powder which is arranged at a distance from thesemifinished part but in a vicinity of the semifinished part.
 15. Themethod of claim 14, wherein a molybdenum silicide or molybdenumdisilicide layer is formed on at least part of the surface of thecomponent as a result of the diffusion annealing or the diffusion heattreatment.
 16. The method of claim 14, wherein after the diffusionannealing or the diffusion heat treatment conditioning of the componentby a high-temperature oxidation at a temperature above 900° C. iscarried out.
 17. The method of claim 16, wherein the conditioning iscarried out at a temperature of from 1000° C. to 1400° C. for from 2hours to 100 hours.
 18. The method of claim 14, alternative (b), whereinthe powder for transferring the silicon via the gas phase comprises oneor more halogens.
 19. The method of claim 18, wherein the powdercomprises one or more of NH₄F, NH₄Cl, and NaF.
 20. The method of claim14, alternative (b), wherein the powder for transferring the silicon viathe gas phase is arranged in a vessel underneath the semifinished part.21. The method of claim 14, alternative (b), wherein the diffusion heattreatment is carried out at a temperature above 900° C.
 22. The methodof claim 21, wherein the diffusion heat treatment is carried out at atemperature of from 1000° C. to 1300° C., with a hold time at thetemperature of from 0.5 to 5 hours.
 23. The method of claim 21, whereinthe diffusion heat treatment is carried out under a protective gasatmosphere.
 24. The method of claim 14, alternative (a), wherein theslip comprises Si powder or Si-containing powder, a solvent and abinder.
 25. The method of claim 24, wherein the binder comprises apolyvinyl alcohol and/or a resin.
 26. The method of claim 14,alternative (a), wherein the slip comprises powder of at least one ofaluminum oxide, zirconium oxide, yttrium oxide, hafnium oxide, neodymiumoxide, silicon carbide, silicon nitride.
 27. The method of claim 14,alternative (a), wherein the slip comprises powder particles having anaverage or maximum particle size of from 0.5 to 100 μm.
 28. The methodof claim 14, alternative (a), wherein the slip is applied by dipping thesemifinished part into the slip, spraying the slip onto the semifinishedpart or printing the slip onto the semifinished part.
 29. The method ofclaim 14, alternative (a), wherein the slip is applied by onto thesemifinished part by screen printing or template printing.
 30. Themethod of claim 14, alternative (a), wherein the diffusion annealing iscarried out at a temperature above 900° C.
 31. The method of claim 30,wherein the diffusion annealing is carried out at a temperature of from1000° C. and 1400° C., with a hold time at the temperature of from 1minute to 3 hours.
 32. A component of a Mo base alloy which is protectedagainst high-temperature oxidation, wherein the component is produced bythe method of claim
 14. 33. The component of claim 32, wherein thecomponent is a component of a turbomachine.